This invention relates to polyphase liquid rheostats, of the type having electrodes arranged in insulated cells filled with an electrolyte which have their tops covered and are opened on end face, in general and particularly to an improved arrangement in such a rheostat which prevents electrical short circuits between phases should a leak occur.
A liquid rheostat is disclosed in German Pat. No. 1,003,835 in which an electrolyte tank having inside walls which are coated with rubber is subdivided by means of rubber coated partitions into cells for the electrodes which are opened at their end face. In front of the end faces room is provided for accommodating a cooling device which is arranged in the electrolyte tank. Should cracks or other faults occur in the easily damaged rubber, a voltage breakdown occurs which puts the entire piece of equipment out of operation.
To overcome such problems it has been proposed to place separate individual cells coated with rubber on the inside of an electrolyte tank in order to permit replacing a damaged cell with a new cell quickly and at low cost.
Because the construction and maintenance of cells coated with rubber is expensive and cells which are stored often suffer cracks in the rubber coating because of temperatture changes, there have been attempts to substitute for the cells coated with rubber, individual cells of an insulating material. Such is disclosed in German design Pat. No. 1,948,381. However, manufacturing such one piece cells requires a considerable expenditure for tooling and, in addition, requires separate mounting and stiffening means within the electrolyte tank to brace the flexible cells appropriately.
In apparatus using a fixed resistance, such as that used for a braking resistor in an electric machine, only a single electrode, fixed relative to the equipment, is provided in each cell and connected to a respective phase. The common neutral point is formed by the electrolyte outside the cells. In liquid rheostats the electrodes can be arranged such that they are adjustable in the horizontal direction, which requires a considerable expenditure of money for the electrical connections and the support members for the adjustable electrodes since the leads must be resistant to high voltage. In order to reduce these costs, it is known in the art to provide, for each cell, an electrode which is fixed relative to the equipment and is connected to an electric circuit, and an unconnected, horizontally adjustable counter electrode, the counter electrodes being electrically connected with each other in a simple manner so that essentially no current flows through the electrodes outside the cells. This is advantageous since it reduces heating and improves the accuracy to which the resistors can be adjusted. The arrangement for adjustment of the electrodes is, however, not the subject of the present invention. This information is provided solely for background. The object of the present invention is to provide an apparatus of the general type mentioned above which is operationally safe and uses cells which are easy to manufacture.